category menu highlighting for single posts in Eleven40 Pro

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My request relates to a category-based menu which is in the primary navigation bar, which in Eleven40 Pro sits right below the header. When you select that category from the menu you, of course, have all the relevant posts appearing below and that menu item appears highlighted (in my case, background set to white, when default is grey). All when and good, but when you select a single post, that relevant menu item is not highlighted. I've shopped around the 'Net, and the only piece of coding I've come across that isn't eons old (this one's from 2010) is here: Dynamic category menu highlighting for single posts

I've followed the instructons to the letter -- that is, placed the following in my functions.php:

//* Filter for single Posts in category menu
add_filter('wp_list_categories', 'sgr_show_current_cat_on_single');

But then those instructions go on to mention finding the right css selector and adjusting the style relevant to your theme. And yet -- using Firebug -- I've tried every configuration I can think of, none of which work. Can anyone help me out? For starters, this seems like a catch-22, because until I know the right selector, I don't even know if the code I've added to the functions.php is even working.

FYI, the css code I've used for highlighting the menu when the user merely clicks on the menu item is this (ie, not for single posts):

I'll try again: I have a category-based menu in the primary navigation bar in Eleven40 Pro. When each menu item is selected it is highlighted, or perhaps I should say "active". But when single posts are selected, they do not "activate" the corresponding menu item. In other words, the category-based URL doesn't highlight in the menu.

For example, click on "Hot Topic" in the primary navigation bar, and it's active/highlighted, like so:

But when a URL within that category is selected (as a single post), the corresponding menu item is not active/highlighted:

The functions code I used above was copied straight in from the above-mentioned article (Dynamic category menu highlighting for single posts). According tho the comments below it, it seems to work fine for others-- although, maybe I accidentally messed with it.

In any case, if that code isn't working/is wrong, what do I need to add to my functions.php? And also, what should I put in my child theme css?

Sorry for the dumbass qu's. I'm fine with using css -- but functions.php is all new to me.

Thanks Brad. Everything I've read on this issue is 5 (or even more) years old. I had a feeling I'd stumbled on a long-known-but-as-yet-unsolved WordPress problem. To echo comments seen elsewhere (others sites), it would seem perfectly logical to want a category-based URL to activate a category-based menu. I was just hoping someone here might have found a way within the Genesis framework. I'll try as you've suggested (with fingers crossed for a solution from any passers-by here!). By the way, the site I grabbed the code from is using a Genesis theme -- so with the "G" favicon appearing in the browser tab, I (oops) mistook it to be a site providing tips for Genesis. Lesson learned! I'll certainly post back here if I ever solve this. Thanks again.